1. Field of the Invention
The present disclosure relates to a shift register, and, more particularly, to a shift register capable of stably generating an output even when the threshold voltage of a pull-down switching element is raised due to degradation of the pull-down switching element.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device is adapted to display an image by adjusting light transmittance of a liquid crystal using an electric field. To this end, the LCD device includes a liquid crystal panel having pixel areas arranged in matrix form, and a driving circuit for driving the liquid crystal panel.
In the liquid crystal panel, a plurality of gate lines and a plurality of data lines are arranged to cross each other, and the pixel areas are defined respectively at intersections of the gate lines and the data lines. Also formed in the liquid crystal panel are pixel electrodes and a common electrode for applying the electric field to the respective pixel areas.
Each of the pixel electrodes is coupled to an associated one of the data lines via the source terminal and drain terminal of a thin film transistor (TFT), which is a switching device. The TFT is turned on in response to a scan pulse applied to the gate terminal thereof via an associated one of the gate lines, so as to charge a data signal on the associated data line into a pixel voltage.
Meanwhile, the driving circuit includes a gate driver for driving the gate lines, a data driver for driving the data lines, a timing controller for supplying control signals for control of the gate driver and data driver, and a power supply for supplying various drive voltages to be used in the LCD device.
The gate driver sequentially supplies scan pulses to the gate lines to sequentially drive liquid crystal cells in the liquid crystal panel on a line-by-line basis. In order to sequentially output the above-mentioned scan pulses, the gate driver includes a shift register.
The shift register includes a plurality of stages each having a plurality of switching elements.
A pull-down switching element, which is arranged at an output unit of each stage, functions to stabilize the associated gate line by a low voltage. To this end, the pull-down switching element supplies a discharge voltage to the gate line after generation of an output at the associated stage. The pull-down switching element should be maintained in an ON state for a large portion of one frame period. For this reason, degradation of the pull-down switching element may be accelerated.
Degradation of the pull-down switching element causes rising of the threshold voltage of the pull-down switching element. In this case, the pull-down switching element may not be turned on, even when a normal voltage is applied thereto.